A printing machine of this type conventionally comprises, in addition to the blanket-carrying cylinder and the blanket, a plate-carrying cylinder and a plate and a pressing cylinder.
During operation, the plate carried by the plate-carrying cylinder is moistened then inked. The plate transfers the ink from the printing regions thereof to the blanket carried by the blanket-carrying cylinder. The blanket then transfers the ink to the paper to be printed which passes between the blanket-carrying cylinder and the pressing cylinder.
In order to tension the blanket on the cylinder, metal bars crimped to the ends of the blanket were traditionally used. These bars were engaged in the aperture of the cylinder, where they were pushed by a screw-type device.
Such arrangements lead to aperture widths which are very large, reaching 22 mm.
The presence of such large apertures results in shocks and vibrations when the blanket-carrying cylinder rotates against the plate-carrying cylinder and against the pressing cylinder. Such vibrations limit the rotation speeds of the various cylinders and therefore the production capacities of offset printing machines.
FR-2 573 347 proposed an assembly of the above-mentioned type. The front edge of the blanket covers all the front edge of the support plate and therefore extends into the cylinder aperture. The rear edge of the blanket does not extend into the aperture and is applied substantially against the region of the front bend of the blanket unit. In this manner, the gap between the start of the blanket and the end of the blanket is extremely narrow in order to limit the vibrations which result from the presence of the aperture in the cylinder.
However, it was found that it was extremely difficult to bend the front edge of the plate and the front edge of the blanket in order to obtain the configuration desired in this document.
FR-2 733 719 proposed another solution in which the device for fixing the blanket unit comprises pressing studs which apply the front and rear edges of the support plate against the front wall of the aperture. These front and rear edges of the support plate are not covered by the blanket. The forces applied by the studs are exclusively perpendicular to the front and rear edges of the support plate.
It was found that this arrangement did not allow the blanket unit to be tensioned satisfactorily. In addition, the fixing device described does not allow the deformations of the blanket unit resulting from the operation of the offset printing machine to be absorbed. In particular, the expansions of the support plate cannot be absorbed so that cracks in the plate adjacent to the rear bend thereof are observed.
FR-2 745 524 proposed that the pressing studs be replaced with one or more resilient blades. However, this arrangement does not always allow satisfactory tension of the blanket unit to be obtained.
An object of the invention is to provide an assembly of the above-mentioned type which allows the blanket unit to be tensioned on and fixed to the cylinder in a satisfactory manner and which is simple and economical to produce.